The present invention relates to an improved facsimile receiving apparatus.
It is known in the art to transmit a synchronization signal for a predetermined length of time prior to transmitting picture signals to a facsimile receiver. The purpose of the synchronization signal is to synchronize the scanning system of the receiver, which typically comprises a rotary drum to which a copy sheet is attached adjacent to which a printing element such as a stylus is provided and means for moving the sytlus longitudinally relative to the axis of the drum, with the scanning speed of the transmitter. Specifically, the synchronization signal may be designed to set the frequency and phase of a pulse generator which produces drum drive pulses. The synchronization circuit may comprise a phase lock loop or similar electronic circuitry designed to lock onto the synchronization signal.
The facsimile receiver comprises the printing element which is responsive to the picture signal and the synchronization circuit which is responsive to the synchronization signal, and also a timing circuit which enables the synchronization circuit to receive the synchronization signal during the time of transmission thereof and subsequently enable the printing element to receive the picture signal. The timing circuit is triggered by the initiation of the synchronization signal and comprises a timer set for a time period equal to that of the synchronization signal.
However, it is difficult in practice to adjust the timers of the transmitter and receiver to precisely the same time period. If the timer of the receiver is set for a period shorter than that of the transmitter, the last part of the synchronization signal will be reproduced by the printing element. If the timer of the receiver is set for a period longer than that of the transmitter, the first part of the picture signal will be received by the synchronization circuit and upset the synchronization.